Conventionally, a scroll blade used in a scroll compressor has been manufactured by a machining center. However, at present scroll blades are machined by a method called simultaneous bi-axial machining in which a cutting tool is linearly moved along an involute reference curve as a work-piece is rotated. Japanese Patent Application Laid-Open Publication Nos. 6-028812 and 2-41847, for instance, disclose examples of such simultaneous bi-axial machining.
In cutting an elongated hole in a work-piece, a central axis of the hole is aligned to a work-piece rotation axis of a grinder while the work-piece is rotated about the work-piece rotation axis. At this stage, a grindstone is rotated at a high speed through a shaft of the grindstone and a spindle rotor. Next, the work-piece is ground by moving the grindstone radially outwardly of the hole. Simultaneously with this, the grindstone is moved along the rotation axis.
Meanwhile, not only in the machining with the machining center but also in the simultaneous bi-axial machining, the tool is supported at its one end away from its cutting edge. Then, the machining of the curved surface in the scroll blade for the scroll compressor causes the tool to deflect or bend when the cutting edge is subjected to a cutting resistance or grinding resistance. The deflection deviates the cutting edge of the tool from a cutting or grinding position on the involute, which in turn changes an angle of the curved surface of the scroll blade to a reference axis. In addition, not only the tool but also the work-piece or the scroll blade tend to deflect due to the cutting or grinding resistance, which causes deterioration in the precision of the machining.
To overcome those problems, the inventor of the present invention discloses, in Japanese Patent Application Laid-Open Publication No. 5-57518, a technique in which the displacement of the cutting edge is calculated in advance and then the cutting edge of the tool is configuration so that the displacement of the cutting edge relative to the work-piece cancels the displacement.
However, the curved surface of the scroll blade has a different radius of curvature depending on a place. Therefore, the deflections of the tool and the work-piece during machining of the curved surface of the scroll blade change in accordance with a position being machined. In view of the deflection of the work-piece, the work-piece tends to cause less deflection at the machining of the central portion with the smaller radius of curvature while it causes larger deflection at the machining of the peripheral portion with the larger radius of curvature. However, in view of the deflection of the tool, the deflection of the tool at the machining of the radially inwardly faced surface of the scroll blade advances in a direction that is opposite to that at the machining of the radially outwardly faced surface thereof.
Therefore, the machining of the radially inwardly faced surface of the central portion of the scroll compressor forms an enlarged length of the contact region of the tool and the work-piece, which causes an enhanced machining resistance and deflection of the tool. Also, as the machining position moves radially outwardly, the contact length decreases to reduce the deflection. Contrary to this, the machining of the radially outwardly faced surface of the scroll blade forms a reduced length of the contact region between the tool and the work-piece, which causes a reduced machining resistance and deflection of the tool. In view of above, ideally tool configuration changes according to the position of scroll blade to be machined.
In order to increase the machining precision only, the tool may be moved slowly relative to the work-piece or, as described in the Japanese Patent Laid-Open Publication No. 2-41846, the contact region between the work-piece and the tool may be extended to reduce the rotational velocity of the work-piece. However, this results in an extension of the machining time and an deterioration of the working efficiency.
Meanwhile, Japanese Patent Laid-Open Publication No. 8-318418 describes a drive system which includes a cam and a cam follower, where the cam is driven by a servo motor so that a flexible trunnion is distorted to rotate a spindle gauge. This drive system, however, due to a reduced rigidity of the trunnion, is considered unsuitable for the high-speed machining.
On the other hand, for the internal grinding of the hole, since a shaft of a grindstone is extremely long, the hole assumes a taper so that an inner diameter thereof decreases as the hole advances inwardly, which deteriorates the configuration precision in terms of cylindricality and linearity. To overcome this, another technique in which the axis of the grindstone or work-piece is intentionally tilted to offset the deflection of the tool is disclosed in Japanese Patent Laid-Open Publication Nos. 61-252064 and 62-166955, for example. However, this technique has a drawback in a responsibility and a resolution for an angle adjustment of the spindle during traversing. Another technique is disclosed in Japanese Patent Application Laid-Open Publication No. 1-240267, in which the spindle rotor axis is tilted by the use of a magnetic bearing, which is considerably costly.